Electric field created p–n junction in composite films made from carbon nanotubes, iron (III) sulfate and polyvinyl alcohol

Suspension made of Fe2(SO4)3(aq), polyvinyl alcohol and carbon nanotubes is placed in electric field to separate charges. Charges remain separated as suspension solidifies, forming composite films with cations and anions enriched at opposite sides. Polarized films behave as junction diodes with forward current and threshold voltage found to be 10–4–10–5 A and 2.4–2.6 V at ± 5 V. Rectification is preserved in strained composite films.

Structural and electrical characterizations of composite films. As made composite films are inspected by optical and scan electron microscope (OM equipped with precision measurement s ystem & SEM, Hitachi-SU8010) which give information regarding to film thickness, surface texture and CNT distribution. The current-voltage (I-V) profiles, electrical conductivity (σ) and resistivity (ρ), normalized ρ at 25 °C (ρ/ρ 25°C ), forward current (I F ), threshold voltage (V th ), revere and forward device resistance (R R and R F ) are measured using a semiconductor device analyzer equipped with mobile-probe system (± 5 V, Agilent B1500A). It is worth mentioning for conventional devices that DL suffers greatly from heating according to Joule's laws expressed as P = VI and I = Q/t where P, V. I, Q and t denote power, voltage, current, thermal energy and time. Accordingly, the I F is defined as the maximum forward current before onset of thermal damage 1 . The R R /R F ratio, on the other hand, is a key parameter often used to characterize electrical rectification, namely, R R /R F ≈ 1 indicates ohmic conductors (i.e. symmetric I-V profile) and R R /R F > 1 for polarized conductors 1 . All electrical measurements are carried out at a low relative humidity (RH = 25%) to evade H 2 O doping since both PVA and CNTs are electronically sensitive to moisture 10 . Elemental analyses. X-ray photoelectron emission spectroscopy (XPS, ULVAC-PHI PHI 5000 Versaprobe II) is employed to probe distribution and content of S and Fe at film surfaces where P and N denote positively and negatively charged sides (iv, Fig. 1b). XPS peak intensity (I t ) is calculated according to full width at half maximum (FWHM) and signal intensity (C x ) of S2p and Fe2p spectra expressed as C x = n x /∑n i = (I x /X x )/∑(I i /X i ) where X x is corrected relative sensitivity factor and I x is signal intensity 11 . The P/N ratio which is based on C x indicate the level of E induced film polarization, i.e. P/N ≅ 1 for non-polar and, P/N > 1 or < 1 for polar.
Work function measurements. The work function (Φ) of composite films is calculated according to Φ = hv-E o -E F where excitation energy (hv), cut-off energy (E o ) and Fermi energy (E F ) are determined by tangential equation and ultraviolet photoelectron spectroscopy (UPS) at hv = 21.22 eV 12 . Supplementary information 2a-c shows UPS spectra where E o , E F and Φ are found to be 13.2 and -3.2 and 4.91 eV. For the sake of clarity, the Φ and UPS are only calculated for samples made of el-Conc = 0.1 and 0.5 M at E = 0 and 600 V/cm (supplementary information 2d-h). The obtained Φ is then used as reference to evade formation of Schottky potential at lead-sample interface, namely, the Φ of metal leads must be lowered and greater respectively than P and N of samples 13 . Study here selects Ag (Φ = 4.26 eV) and Pt (Φ = 5.65 eV) paste as metal leads in connection with P and N.

Molecular dynamic calculations. Molecular dynamic (MD) calculation is performed to characterize E
induced charge separation in suspension and, single-walled CNTs (SWCNTs) are used as modelling matrix for the following reasons. First, less carbon atoms are invovled so calcualtion time is shortened. Second, SWCNTs resemble MWCNTs in terms of adsorption, transport, chemical reactivity and optical properties 14 Figure 3a displays a typical example of composite films with thickness measured to be 1 mm. Clearly, CNTs are well-dispersed and mostly embedded according to SEM images obtained from different regions of cross-section ( Fig. 3b-c). The log(σ)-f CNT plot indicates f CNT = η = 10-12 wt% at which the σ is 2-3 orders of magnitude greater than value obtained at f CNT = 2.5 wt%  www.nature.com/scientificreports/ (Fig. 3d). The small fluctuation of ρ/ρ 25°C with rising temperature at RH = 25%, however, excludes H 2 O doping (insert, Fig. 3d) 10 . Figure 4a-d shows XPS profiles where P-Fe2p, P-S2p, N-Fe2p and N-S2p denote Fe2p and S2p spectra recorded at P and N of composite films made from el-Conc = 1 M and E = 30 V/cm. The Fe2p 1/2 and Fe2p 3/2 are present in P-Fe2p; the former consists of Fe 3+ multiplets (singlet, 722-730 eV) and Fe 2+ satellite for the latter (triplet, 707-717.5 eV, Fig. 4a) 15 . We find that CNT addition truly promotes ion absorption thus giving a greater I t compared with CNTs-free samples (top and lower, Fig. 4a). Detection of Fe 2+ , on the other hand, indicates Fe 3+ → Fe 2+ reduction through CNTs acting as reductant 16 . Similar profiles also appear at N with (i) I t(P) < I t(N) for CNTs-free (lower, Fig. 4b) and (ii) I t(P) ≈ I t(N) for CNTs loaded films where I t(P) and I t(N) denote XPS peak intensity at P and N (top, Fig. 4b). The (i) implies that cations move with E and become enriched at N. The (ii) is due to ion-tube physisorption thus reducing mobility of ions toward electrodes 8 . In S2p spectra, sulfates (SO 4 2− ) appear in the form of doublet [167.9 eV (S2p 3/2 ) and 169 eV (S2p 1/2 )], attributable to spin-orbit interaction (Fig. 4c-d) 15 . Again, I t(P) ≈ I t(N) and I t(P) < I t(N) are present respectively in films with and without CNT addition (top and lower, Fig. 4c and d). Table 1 Figure 5a shows I-V profiles of composite films made from PVA loaded respectively with electrolyte (el-Conc = 1 M, sample 1) and CNTs (f CNT = 12wt%, sample 2) at E = 30 V/cm. Addition of both electrolyte and CNTs into PVA yields sample 3 (i.e. samples 1 + 2). Clearly, samples 1 and 2 exhibit a linear I-V character; the latter due to networking of CNTs at η exhibits a lower R (insert, Fig. 5a) 18 . Sample 3, on the other hand, resembles a diode (insert, Fig. 5a). Diode character is enhanced as el-Conc of sample 3 is reduced to 0.5 M (defined as sample 4, insert, Fig. 5b). Data above indicates following, first, CNTs provide samples with σ (i.e. sample 2), second, diode character is only present in samples made from electrolyte, CNTs and E treatment (i. e. samples 3 and 4). Third, diode profiles vanish when CNTs or electrolytes are removed from sample 3 (i. e. samples 1 and 2). Fourth, the optimal el-Conc for diode production is 0.5 M (i.e. sample 4). I-V measurements are then carried out on samples 5 and 6 to probe optimal E for diode production where 5 and 6 denote composite films made by addition of el-Conc = 0.1 and 0.3 M respectively into sample 2 (supplementary information 4a-b). Clearly, diode character reoccurs in samples 5 and 6 with I F measured to be 310 μA and 75 μA at 5 V (E = 600 V/cm). Highlighted I-V profiles of sample 6 further reveal an enhanced diode character where V th lies 2.4-2.6 V at E = 600 V/cm (supplementary information 5a-c). Note that DL is a polarized region with field vector against external E the I F is therefore expected to be lower than a conductor, accounting for I F(E = 0V) = 120 μA > I F(E = 30V) = 85 μA > I F(E = 600V) = 75 μA at 5 V (supplementary information 5a-c). Additional evidence in support of film polarity comes from the Φ measurements (Table 2). First, Φ P(600 V) > Φ N(600V) is present where Φ P and Φ N denote Φ obtained at P and N. Second, Φ rises as el-Conc increases from 0.1 to 0.5 M, indicative of enriched ions at surfaces. However, the Φ varies significantly from region to region as el-Conc reaches 1 M (not shown), attributed to excessive ions that diffuse randomly thus compromising DL formation 17 . Supplementary information 6 shows stability of diode characteristic over 12 months kept in the oven.

Results and discussion
Diode performance is often justified according to quality factor and R R /R F ratio 19 ; the former is used to characterize doped devices and is inapplicable to CPs. The latter concerns unidirectional conduction so V th must low and high I F is required, for example, V th = 0.7 V and I F = 10 -3 -10 -5 A for Si-based diodes 20 . Experiments indicate that diode character becomes apparent at R R /R F > 3 and, the V th and I F lie 2.4-2.6 V and 10 -4 -10 -5 A for samples made at E = 30 and 600 V/cm (Table 3 and Fig. 5c-e). The R R /R F measurements also confirm optimal conditions for diode formation to be f CNT = 12 wt%, el-Conc = 0.5 M and E = 600 V/cm; the greatest value being R R /R F = 20.663 (Table 3). In CMOS technology, the Si substrates are intentionally stressed to break crystallographic symmetry thus altering band structure 21 . However, strains are confined at interfaces and can barely propagate to detriment conductive channels underneath. We find that film area and thickness increase and decrease respectively by 200% and 80% after hot-pressing (60 °C, 50 kg/cm 2 and 30 min) and the R R /R F changes to 3.176 (E = 0), 2.358 (E = 30 V/m) and 4.03 (E = 600 V/m), indicating redistribution of ions and CNTs (Table 3 and supplementary  information 7a-b).
Question however remains as to how charges separate in the presence of E, CNTs and PVA. Supplementary information 8 shows consecutive snapshots extracted from MD simulations of complex-I which contains three   In the absence of E, the displacing range of ion-ion clusters is 3-4 nm and decreases to 1-2 nm as E is applied, attributed to static attraction by electrodes (supplementary information 10a-b). PVA however acts as impermeable objects and prevents cations and anions from pairing, thus facilitating charge separation (complex-II, supplementary information 11a-c and 12a). Calculation indicates the number of cations and anions to be 3 and 2 at N and 4 and 7 at P, corresponding to 5 positive charges at N and 2 negative charges at P (t = 6 s, supplementary information 11d-f and 12b). H-bonds also exist between PVA and anions, accounting for simultaneous movements of both structures toward electrodes (i.e. CH 2 -CHOH⋅OSO 3 2− , circle, supplementary information 9c). Owing to large mass, CNTs can barely move and therefore restrict movements of PVA and ion clusters (complex-III, Fig. 7a-b and supplementary information 13a). Aggregation of ions around tubes again verifies physisorption. CNTs however become polarized (white N and P) as E is applied and therefore attract more ions, resulting in I t(tube) > I t(tube-free) (top and lower, Figs. 4a, 6c,d and supplementary information 13b) 22 .

Conclusion
Diode structure is successfully created in composite films made of PVA, Fe 2 (SO 4 ) 3(aq) and CNTs. DL and electrical rectification are verified by elemental analyses and I-V measurements. Calculations confirm E induced film polarization and charge separation through hydration with H 2 O acting as carriers. Diode character is preserved in strained composite films. www.nature.com/scientificreports/